The present invention relates in general to a notch filter and in particular to an improved notch filter which has excellent rejection at the notch frequency and has low insertion loss across a very wide pass band. The notch filter utilizes a quadrature coupler with the input signal on one of its four terminals, a first tuned surface acoustic wave device coupled to a second one of the terminals for receiving the input signal at 0.degree. degree phase shift, a second tuned surface acoustic wave device coupled to a third one of the terminals for receiving the input signal with a 90.degree. phase shift and the fourth terminal being utilized as the output signal terminal.
A notch filter is a device which is generally presumed to pass all frequencies except for a very narrow band which is "notched out". This type of filter is also sometimes called a band elimination filter. In practice, the notch filter will have only a limited band for passing signals but typically this band width is several octaves wide. These filters are widely used to eliminate discrete frequency regions where unwanted carriers appear from adjacent channels or other sources. In addition, they are used to deny access to certain signals or alternatively to remove interference in some signals.
Typically, it is desired that a notch filter have the lowest possible insertion loss with no ripple in the pass region, and as much rejection as possible across the notch region.
Notch filters are generally implemented with inductors and capacitors and can take one of many forms. However, they are prone to temperature drift and have poor Q at UHF frequencies. The net result is that notch filters implemented using LC components tend to drift off the desired frequency and in many cases the notch width is too broad or the filter loss is too high or both. For frequencies above approximately 150 MHz, notch circuits are generally not utilized because of these problems.
Surface acoustic wave (SAW) devices inherently have much higher Q's than LC filter elements and further they generally exhibit much superior temperature stability as compared with LC elements. Thus it is desirable to implement notch filters using SAW devices particularly in the UHF band.
The difficulty in implementing a notch filter with SAW devices stems from the fact that SAW devices are of themselves band pass elements in their normal configuration in which one transducer launches a wave and a second transducer receives the wave. Thus to produce a notch characteristic, the devices must be used in conjunction with some other circuit which converts the band pass region of the SAW to a band reject region and vice versa.
One such notch filter is disclosed in U.S. Patent No. 4,577,168 in which a first circuit incorporates a single-port, single-phase, unidirectional surface acoustic wave transducer (SPUDT) as the impedance element coupled to and parallel with an inductor whose value is chosen to resonate with the static capacitance of the transducer at the center frequency of the notch. This means that this first parallel circuit is primarily reactive except at the notch frequency where the circuit is primarily resistive in nature. By placing a resistance in parallel with the first parallel circuit to form a second parallel circuit and choosing a value of the resistance substantially equal to the resistive value of the first parallel circuit at the center of the notch frequency, and by placing a phase reversal transformer in series with either the resistance or the first parallel circuit, the total circuit provides equal and opposite cancellation of signals at the notch frequency but allows all other frequencies to pass thus creating a notch filter.
This circuit functions well as a notch filter but has an inherent problem. In the prior art circuit, the SAW device is being used as a transmission element. In order to have maximum rejection of the signal frequency in the notch, the parallel inductor-impedance element circuit must become resistive and be equal in value to the resistor forming the second parallel circuit. The resistor forming the second parallel circuit must be chosen to have a value equal to 2Z.sub.o, where Z.sub.o, is the input load, the insertion loss in the pass band is flat. This value, 2Z.sub.o, for the resistance causes the circuit in the notch region to be perfectly matched to the source, Z.sub.o. Thus there is total absorption of the power across the notch region.
However, at frequencies in the pass band, the parallel inductor-impedance element circuit becomes essentially purely reactive or an open circuit. This means that the signal is passing only through the resistance forming the second parallel circuit. However, that resistance is now twice the value of the resistance of the input load thus creating a mismatch. Because of the mismatch there is a reflection loss and there is a dissipation loss in the resistance resulting in a net insertion loss of GdB in the filter. This insertion loss cannot be eliminated with this type of circuit because the notch filter circuit is actually the equivalent of two parallel resistors at the notch frequency and in order obtain a flat response in the pass band, they must be double the value of the input load. In the band pass region, the notch filter consists only of the resistance of the second parallel circuit, which, alone, is twice the value of the input load.
The reflections from the notch circuit in the pass band and the minimum GdB insertion loss are both undesirable.
The present invention overcomes the disadvantages of the prior art by providing a quadrature coupler which has first, second, third, and fourth terminals, a first terminal being used as a signal input terminal and a fourth terminal being used as the output terminal. The parallel inductor SAW circuit is coupled as a load to a second one of the quadrature terminals for receiving the input signal with a 0.degree. phase shift. The load has a primarily reactive characteristic at a signal frequency higher or lower than the notch frequency band width (in the pass band) and primarily resistive characteristics across the notch region as described in the prior art. A second load is coupled to a third one of the quadrature coupler terminals for receiving the input with a 90.degree. phase shift. This load is also primarily reactive at signal frequencies in the pass band and is primarily resistive across the notch region. When an input signal is applied to the input terminal at a frequency in the notch region, both of the loads coupled to the second and third terminals are primarily resistive and matched to the source impedance and simply absorb the load thus providing no output in creating the notch.
However, in the band pass region a different operation occurs. At the second port or terminal which is receiving the input signal with a 0.degree. phase shift, the load, in the band pass region, is now primarily reactive. Thus the input signal is reflected. It is reflected back to the input terminal with a 0.degree. phase shift and to the fourth or output terminal with a 90.degree. phase shift.
The load which is coupled to the third port or terminal is also primarily reactive and thus reflects the 90.degree. phase shifted signal back to the input terminal shifted the 90.degree. so that it is again 0.degree. at the input terminal. However, it is coupled directly (with 0.degree. phase shift) to the fourth or output terminal and remains the 90.degree. phase shifted signal thereby adding with the 90.degree. phase shifted signal being reflected from the second terminal. Thus the input signal has been transferred to the output terminal in the band pass region with very little insertion loss and yet provides high signal rejection across the notch region thus having all the characteristics of a good notch filter.
Thus it is an object of the present invention to provide an improved notch filter for rejection of a predetermined band of signal frequencies from an input signal and a minimum of insertion loss across a wide pass band region.
It is still another objection of the present invention to provide an improved notch filter which utilizes a quadrature coupler having first and second loads coupled respectively to second and third of the terminals of the quadrature coupler which loads are formed from SAW impedance elements and which are primarily resistive at the notch frequency and primarily reactive in the band pass region thus providing an output signal on the fourth terminal with little insertion loss.